Until relatively recently, insulation of the windings to both superconducting and resistive electromagnets has generally been considered indispensable. However, except for ensuring a specific current path within a winding, insulation is undesirable in several aspects. First, the insulation, generally organic, makes a winding elastically soft and increases mechanical strain of the winding under a given stress, known as the spongy effect. Second, insulation reduces the overall current density of the winding. Third, insulation electrically isolates every turn in a winding and prevents, in the event of a quench, current bypassing through the adjacent turns, which may cause overheating in the quench spot. Therefore, use of thick stabilizer, typically copper (Cu), to protect superconducting magnets from permanent damage is common, resulting in large magnets.
In general, niobium-titanium (NbTi) magnets for magnetic resonance imaging (MRI) must undergo a training sequence when first energized at the manufacturer site. During the training sequence the magnets reach the design operating current after having experienced one to six premature quenches. Typically a whole-body MRI magnet consumes 2000 liters of liquid helium (LHe) during a training sequence. In 2011, GE Medical used five million liters of LHe at their factory for approximately 2000 units of whole-body MRI magnets delivered to the users. Combined with the rising LHe price, which has quadrupled over the last ten years and extra man-hours spent to achieve the magnet operating current, the training sequence adds to the magnet manufacturing cost. Minimizing the number of premature quenches, or even eradicating them, has remained a major challenge during the forty years since a superconducting magnet was first introduced.
NbTi wires for superconducting magnet applications generally contain a significant amount of stabilizer to satisfy stability requirements of superconducting magnets. The stabilizer is typically copper, in the form of a matrix. A typical superconductor-to-copper ratio of NbTi wires for nuclear magnetic resonance (NMR)/MRI magnets is 1:7 or even lower. In contrast, NI (No-Insulation) windings use NbTi/Cu wire bare, un-insulated, so that each NbTi/Cu turn in the NI winding can share the copper stabilizers of its neighbor turns and layers. This copper-sharing allows reduction in copper in the wire without detrimental effects on magnet stability. This reduction in copper in turn beneficially reduces the magnet weight. The NI technique has been analytically and experimentally shown to be applicable to full-scale NMR/MRI magnets.
FIG. 1 shows a schematic drawing of a prior art magnet 100 detailing an m-turn by p-layer (m×n) NI winding of a coil 105. As depicted by FIG. 1, the first (innermost) layer 171 is on the left and the last (outermost) layer 176 is on the right. The first layer 171 is adjacent to the cylindrical surface of a bobbin 190. Similarly, a first turn 161 is on the top and a last turn 164 is on the bottom of the coil 105. The first turn 161 and the last turn 164 are adjacent to raised rims of the bobbin 190. The bobbin 190 is not generally depicted in FIG. 1, other than indicating the C shaped profile of the bobbin 190.
The core 130 of each winding 120 is formed of a superconductor material surrounded by a cladding 140 of copper or a copper alloy. Other stabilizers may be used, for example, but not limited to brass, silver, Cu—Ni alloy and aluminum. The “+” symbol indicates a current ingress winding, and the “−” symbol indicates a current egress winding. Contact points 150 between adjacent windings 120 are represented as resistors, indicating that leak current may traverse the contact points 150. The average contact resistances between turns and layers may be modeled as an (m−1) by (n−1) resistor matrix.
In general, magnet protection, for example, from over-heating in an event of quench, is one of the major factors that limits magnet current density. While the NI technique provides several advantages over insulated windings of the prior art, in some circumstances there may be disadvantages. With insulated wire windings, the current follows the spiral coil path of the windings. With NI windings, at least at start-up, current may leak between adjacent bare windings. This leak current may be modeled as an inductor having inductance Lcoil in parallel with a resistor having resistance Rc. Lcoil represents an NI coil inductance, while Rc represents chiefly contact resistances between the bare wires. The model characterizes the non-spiral (i.e., radial and axial) current paths through the contacts within the winding. Non-infinite Rc can leak current to adjacent turns and layers, creating two undesirable issues in the NI coil that only manifest under time-varying conditions when the magnet is charged (or discharged): delay in charging time and ohmic loss in the winding. The delay in charging time may result in considerable cost, due to consumption of additional coolant, such as liquid helium. Therefore, there is a need in the industry to overcome the abovementioned shortcomings.